A number of formats exist for video signals, including television signals. For example, conventional television in the United States uses the 525 line/60 Hertz National Television System Committee (“NTSC”) standard, while Europe uses a 625 line/50 Hertz standard. Television formats in the United States include 480i, 480p, 720p, and 1080i, wherein the number indicates the number of scan lines, “i” indicates an interlaced signal, and “p” indicates a progressive scan signal. Motion pictures are recorded on film at 24 frames per second (i.e., 24 Hertz).
For an interlaced television format, each frame is displayed as two sets of spaced lines that are successively scanned. One set of lines comprises even-numbered lines and the other set of lines comprises odd-numbered lines. Each set of spaced lines is referred to a field. The persistence of vision makes two interlaced successive fields appear to be a frame. The phrase “vertical period” as used herein means a field for an interlaced video format and a frame for a non-interlaced video format.
FIG. 1 shows an example of a prior art video system that includes a video cassette recorder (“VCR”) 2, a video decoder 6, and a video processor 10. Video decoder 6 digitizes the analog video output signal from VCR 2 to form a digital output signal that includes a data signal and a clock. A phase lock loop (“PLL”) circuit 5 of video decoder 6 generates a clock signal from a horizontal synch signal that is part of the analog output of VCR 2. The clock signal and the digital output signal from video decoder 6 are sent to video processor 10.
Prior art video processor 10 converts video data and clock signals from one format to another format using phase-lock loop circuitry 7 that locks to the input clock. For example, the video processor 10 can receive a digital video signal in a 480p format and covert the signal to a 720p format. The frequency of the output clock is proportional to the frequency of the input clock. For the example, the input and output clocks are pixel clocks. For a stable 480p input signal, the frequency of the input signal is 27 megahertz. For a stable 480p input signal, the frequency of the output clock for a 720p output signal is 74.1 megahertz.
Unfortunately, input video signals are not always stable. For example, prior art video cassette recorders (“VCR”) typically produce unstable video signals. Typically, VCRs have very unstable horizontal synch signals and somewhat unstable vertical synch signals. The unstable VCR signals have varying pulse widths and varying frequencies.
One problem with the prior art video system of FIG. 1 is that an unstable video input signal produces an unstable video output signal. For example, if the horizontal synch signal that is provided as an output from VCR 2 has a varying frequency, then the clock output of video decoder 6 will have a varying frequency because the PLL circuitry 5 locks to the horizontal synch signal of VCR 2. Likewise, the output clock of video processor 10 will have a varying frequency when the output clock of video decoder 6 has a varying frequency, given that PLL circuitry 7 locks to the output clock of video decoder 6. This can result in a video signal with poor stability at the output of video processor 10.
Various prior art techniques have been used to convert a motion picture 24 Hertz signal to a television signal. For example, in the United States, a 3:2 pulldown technique has been used to convert the 24 Hertz motion picture signal to a 60 Hertz non-interlaced (progressive scan) television signal. FIG. 2 illustrates the prior art 3:2 pulldown technique. Each film frame is tripled or doubled in an alternating pattern to “fit” within the 60 Hertz non-interlaced (progressive scan) television signal. One disadvantage of the 3:2 pulldown technique is that uneven motion is produced given that the film frames appear in a pattern of 3 frames, then 2 frames, then 3 frames, etc.
Europe uses a different technique for converting motion picture signals. Video processors in Europe speed up the 24 Hertz motion picture rate to a 25 Hertz rate, which is then converted to a 50 Hertz rate, which is the European standard television rate. One disadvantage of this European technique is that motion pictures appear 4% faster on European television.